1. Field of the Invention
The invention relates to microwave heating and means for producing a browned and seared surface.
2. Description of the Prior Art
Microwave heating has now become widely accepted for the preparation of industrial as well as domestic products. Electromagnetic waves at microwave frequencies are radiated within an enclosure from an energy source such as a magnetron. The microwaves are radiated and reflected within the enclosure in free space and are distributed by such means as mode stirrers and the like to uniformly surround and be absorbed by the load within the enclosure. The microwaves set up high frequency oscillatory movements of the molecules in the load to cause heating by molecular friction. The allocated frequencies for the microwave apparatus are assigned by federal regulatory agencies and are 915.+-.13 MHz and 2450.+-.50 MHz. The term microwaves is defined as electomagnetic energy radiation having wavelengths in the order of 1 meter to 1 millimeter and frequencies in excess of 300 MHz.
Materials exposed to microwave radiation have differing dielectric constant and loss tangent characteristics which results in heating in a varying pattern. As a result the varying absorption of the radiated energy causes the depth of penetration and surface coloration of the load to vary. Desirably a browned surface, similar to that produced by broiling, should be produced in loads radiated with microwave energy without requiring long exposure which results in overcooking of the interior regions. A browned and seared surface is difficult to attain without long exposure because the microwave oven enclosure is cooler during cooking compared to the electric and gas type prior art devices. Typically a surface temperature of approximately 350.degree. to 400.degree. F is required for a browned and seared surface. Prior art techniques for browning include the incorporation of electric or gas broiling elements in the microwave oven. Another method involves the coating of the outer surfaces with a food additive having a high energy absorbing characteristic which will lead to more rapid heating of the outer surfaces. Still another example of prior art teachings involves the use of lossy oven ware or utensils having selective heating capability by means of the use of conductive materials. Such conductive materials become heated from the absorbed energy and transfer this energy thermally to the supported load to function as a heat exchanger. Conductive materials have also been incorporated in a shelf of a dielectric material supporting a load within a microwave oven.
Still another example of a prior art teaching is the utensil described in U.S. Pat. No. 3,857,009 issued Dec. 24, 1974 to G. MacMaster et al and assigned to the assignee of the present invention. This structure incorporates a load supporting means for converting and transforming the free space energy waves into a fringing electric field pattern having a substantially 180.degree. phase differential in close proximity to the exterior surfaces of the load. The means for converting and transforming the planar energy comprises alternating regions of high and low dielectric constant material. While the embodiments of this structure are efficient, easy to clean and relatively light weight, the cost of providing such high dielectric constant materials as those having a value of K38 or K50 tends to be costly. Further, in the patented structure the waves are directed through the dielectric material in varying degrees to evolve the 180.degree. phase differential pattern. The 180.degree. phase differential requires the use of plates having a predetermined height, typically, one-quarter of a wavelength and in certain dielectric materials this dimension can be higher than desired.
A continuing need is evidenced in the microwave heating field for new and improved means for providing the browned and seared surfaces particularly in such items as steaks, chops, fish fillets and the like.